Regional geochemistry and continental heat flow: implications for the origin of the South Australian heat flow anomaly

Abstract

Existing measurements from South Australia define a broad (> 250 km wide) zone of anomalously high surface heat flow (92 ± 10 mW m-2). This zone is centred on the western margin of the Adelaide Fold Belt (Neoproterozoic to early Phanerozoic cover floored by Palaeoproterozoic to Mesoproterozoic basement), where it borders the eastern Gawler Craton and Stuart Shelf (Palaeoproterozoic-Mesoproterozoic). To the west, in the western Gawler Craton (Archaean to Palaeoproterozoic), heat flow averages ~ 54 mW m-2 while to the east in the Willyama Inliers (Palaeoproterozoic) heat flow averages ~ 75 mW m-2. We use a regional geochemical dataset comprising > 2500 analyses to show that the anomalous heat flow zone correlates with exceptional surface heat production values, mainly hosted in Palaeoproterozoic to Mesoproterozoic granites. The median heat production of Precambrian 'basement' rocks increases from < 3 μW m-3 west of the anomalous zone to ~ 6 μW m-3 within the anomalous zone. In the highest known part of the heat flow anomaly, Mesoproterozoic gneisses and granites of the Mount Painter Province in the northern Adelaide Fold Belt yield an area-integrated mean heat production of 9.9 μW m-3. These data suggest that the anomalous heat flow reflects an unusual enrichment in U and Th in this part of the Proterozoic crust, with the total complement of these elements some 2-3 times greater than would be expected for Proterozoic crust on the basis of the global heat flow database. This extraordinary enrichment has played an important role in modulating the thermal regime of the crust in this region, and particularly its response to tectonic activity. (C) 2000 Elsevier Science B.V. All rights reserved.